Bluetooth: Move hci_amp_capable() function into L2CAP core

The hci_amp_capable() function has only a single user inside the L2CAP
core. Instead of exporting the function, place it next to its user.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>